Rubocop: Fix Bundler/OrderedGems and Performance/FluentdPluginLogStringInterpolation

[Watson1978]

This patch will fix following Rubocop's suggestions:

$ bundle exec rubocop
Inspecting 5 files
C...C

Offenses:

Gemfile:8:1: C: [Correctable] Bundler/OrderedGems: Gems should be sorted in an alphabetical order within their section of the Gemfile. Gem rubocop-fluentd should appear before test-unit.
gem "test-unit-rr", "~> 1.0.5" ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Gemfile:9:1: C: [Correctable] Bundler/OrderedGems: Gems should be sorted in an alphabetical order within their section of the Gemfile. Gem rubocop-fluentd should appear before test-unit-rr.
gem "rubocop-fluentd", "~> 0.2.0"
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/fluent/plugin/in_fluent_package_update_notifier.rb:75:11: C: Performance/FluentdPluginLogStringInterpolation: Use log.error { "..." } instead of log.error("...")
          log.error "Failed to check updates: #{e.message}"
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

5 files inspected, 3 offenses detected, 2 offenses autocorrectable